This COM port can then be accessed as if it were a regular RS-232 port.
The LEDs in the adapter will blink when information is being sent through the adapter. This is a
good way to tell if your communication link is active.
RS-232 settings
In order for the computer to communicate with the DC Load, both must be set to the same RS-232
settings. These communication settings are:
1. Baud rate must be one of 4800, 9600, 19200, or 38400.
2. 8 data bits.
3. One stop bit.
4. No parity.
Overview of DC Load programming
Packet structure
The DC Load is programmed using packets of bytes. A packet always contains 26 bytes, either
going to or coming from the instrument. The basic programming rule is:
You send a 26 byte packet to the instrument. You then
read a 26 byte packet back from the DC Load to either
● Get the status of your submitted packet, or
● Get the data you requested.
The following are conventions we will follow in this chapter:
1. Hexadecimal integers will be represented by the prefix 0x.
2. Numbers are in base 10 number system unless otherwise indicated.
3. Byte numbering is zero-based, meaning numbering starts with 0.
The structure of each 26 byte packet is:
Byte 0 Byte 1 Byte 2 Byte 3 to 24 Byte 25
0xAA Address Command Command's data Checksum
Thus, the first byte of any command packet or returned packet is always 0xAA.
Address must be a byte that is between 0x00 and 0xFE. Setting of the address is optional. It is not
required to communicate with the instrument. The address can be set from the front panel and is
stored in non-volatile memory. This feature is useful when communicating via USB, and connecting
several instruments, e.g. via a USB hub. In this scenario, Windows assigns a virtual COM port to
each device which is unknown prior to establishing communications with the instrument (could be
different each time). In this case, the user can correlate each virtual COM port randomly assigned by
Windows with a user defined address.
Command is a byte that identifies which DC Load command is used.
The area for the command's data contains parameter information for the command or the data that
is requested via a previous command. Some commands have no data at all. It is a good
programming practice to set all unused bytes to 0x00.
8500 DC Load Series Version: February 4, 2009 Page 45 of 76
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